Your kinks need fixing.

The first looks hand drawn. The lines added. The single tone fill with darker shade airbrush for shadow and the white airbrushed for highlights.

So You could. Make your outlines using the vert pen/Paths tool. Fill it in and do the shadow/highlight airbrushings.

I also thought that Gimp could use PS filters. But also had edge detect, combined with other filters to give that effect. Using a duplicate layer set to gel/luminosity or what ever to get a toon look can also work. A combination of it all.